General Election 2015: The 75 seats that will decide who governs Britain

The collapse in Lib Dem support, the neck-and-neck polling for Labour and the Tories and the growth of 'insurgent' parties means there are many constituencies to watch

The opinion polls have consistently presented us with three key messages during the election campaign. First, whereas five years ago the Conservatives enjoyed a seven-point lead over Labour, now the two parties are more or less neck and neck. Second, the Liberal Democrats have fallen back heavily from the 24% they enjoyed in 2010. Third, voters appear set to vote for parties other than these three in unprecedented numbers.

This means in turn that there are three key battlegrounds. The first comprises the seats the Conservatives won narrowly in 2010, doing so in most cases by defeating the incumbent Labour MP. They range from seats like Warwickshire North, where the Tory majority last time was just 54 votes, to one like South Ribble where the 5.4% swing Labour requires is above what most polls anticipate the party will achieve.

Which party emerges with most seats – and thus who might be better placed to be prime minister – will be determined by how many of these seats stay blue or are turned red. Tory hopes in many of these rest on the belief that the swing against the party will be lower than it is nationally because many a first term Tory MP will pick up a personal vote as a result of their assiduous efforts during the last five years as constituency MPs.

The experience of first term MPs at other recent elections suggest that expectation may well be right – creating the paradox that who governs Britain could be decided by how voters in marginal seats have reacted to the personal performance of their local MP.

The apparent collapse in the Liberal Democrat vote defines our second battleground – seats the party is trying to defend. The party’s task seems most difficult in those seats, such as Burnley, where the party is trying to head off a Labour challenge. Labour itself is more popular than five years ago, while voters in such seats often appear particularly unforgiving of the Liberal Democrats’ involvement in the coalition.

Even so, the Conservatives are enviously eyeing the 38 Liberal Democrat seats in which they start off in second place, as wining half or so of them potentially provides David Cameron with an alternative route to an overall majority. The Liberal Democrats believe the local popularity of many of their MPs will enable them to withstand the adverse electoral tide. That makes the outcome in traditional Liberal seats such as North Devon vital for both parties.

Finally, but by no means least, are the seats the “insurgents” hope to win. In the case of the SNP this means more or less the whole of Scotland - even Gordon Brown’s former Kirkcaldy and Cowdenbeath seat is apparently not safe. The better the nationalists do, the more fraught and contentious the process of forming a government is likely to be. At the same time, Nigel Farage has staked his future as Ukip leader on winning Thanet South while the Greens have their eye on Bristol West.

Still, at least we – and they - will all know their fates by tomorrow.

John Curtice is Professor of Politics, Strathclyde University